1. Field of the Invention
The present invention pertains to the field of transducers, and more particularly to methods of applying a matching layer to a transducer.
2. Background
Piezoelectric transducers find a wide variety of application in ultrasonic and electroacoustic technologies. Characterized by the presence of a shaped, piezoelectric material such as, for example, lead zirconate titanate (PZT), these devices convert electric signals to ultrasonic waves, and generally vice versa, by means of the piezoelectric effect in solids. This effect is well known in the art of transducers and their manufacture. A piezoelectric material is one that exhibits an electric charge under the application of stress. If a closed circuit is attached to electrodes on the surface of such a material, a charge flow proportional to the stress is observed. A transducer includes a piezoelectric element, and if necessary, an acoustic impedance matching layer and an acoustically absorbing backing layer.
Transducers can be manufactured according to conventional methods. Thus, a thin piezoelectric substrate is metalized on its two surfaces with a coating such as, for example, gold plating. The thickness of the piezoelectric element is a function of the frequency of sound waves. One surface of the piezoelectric element can be coated with an acoustic impedance matching layer, or multiple matching layers, as desired. A backing layer is attached to the backside of the piezoelectric element. The backing layer material is typically cast in place via a mold such that the piezoelectric element lies between the matching layer and the backing material. The matching layer, which may be formed of an electrically conductive material, serves to couple between the acoustic impedances of the piezoelectric element and the material targeted by (i.e., at the front of) the transducer. Individual piezoelectric transducers are made from the piezoelectric-material/matching material-layer. A preferred backing material and method of applying the backing material to a transducer are disclosed and described in related U.S. patent application Ser. No. 09/171,747, entitled Transducer Backing Material And Method Of Application, filed on the same day as the present application and fully incorporated herein by reference.
The method of applying the matching layer must be tailored to result in a precise thickness and acoustic impedance for the matching layer in order to match as closely as possible the acoustic impedance of the piezoelectric material to the acoustic impedance of the medium to which the piezoelectric material is ultrasonically coupled. Conventionally, the matching layer has been applied from above to a surface of the gold-coated, or gold-over-nickel-coated, piezoelectric material. A cylindrically shaped bead of epoxy was positioned at an edge of the surface and then "rolled" on with the aid of a stencil and a doctor blade to form a "smoothed-on" matching layer.
However, a problem with the conventional method is that the outer layer forms a "skin" during preparation before the bead is "rolled," and the skin portion does not stick properly to the piezoelectric material. This results in a "skin effect," i.e., patches on the piezoelectric surface where the matching layer has not adhered. Additionally, such a method often causes air bubbles to become trapped in the matching layer as the epoxy cured. This results in inefficient transducers because the air bubbles reflect ultrasonic waves propagating through them to a degree sufficient to significantly degrade the impedance match.
To reduce the formation of air bubbles, the matching layer has been applied in a vacuum chamber. However, the vacuum tends to increase the skin effect and misshape the bead of epoxy, necessitating that a substantial portion of the bead be discarded during the application process. Hence, the transducer manufacturing process was rendered more costly and less efficient. Moreover, small air bubbles still remained regardless of the vacuum, having been caused by extrusion of the bead from the syringe. Thus, there is a need for an application method that minimizes skin effect and air-bubble formation in the matching layer without increasing the manufacturing cost of the transducer.